MASH1 activates expression of the paired homeodomain transcription factor Phox2a, and couples pan-neuronal and subtype-specific components of autonomic neuronal identity

Correct coordination of neuronal differentiation events in ventral forebrain requires the bHLH factor MASH1

MASH1 is a bHLH transcription factor specifically expressed in the developing nervous system that has an essential role in the formation of multiple neuronal lineages in the peripheral and central nervous systems. Here we demonstrate the requirement for MASH1 for normal development of ventral forebrain structures. MASH1 is expressed at high levels in the ventral telencephalon and specific regions within the ventral diencepharon. In the absence of MASH1, tissue morphology, proliferation, and gene expression within these forebrain regions is disrupted. The decreased incorporation of BrdU in the neuro-epithelium and the enlargement of the ventricles demonstrate a reduction in cell proliferation. A loss of anatomically distinct lateral and medial ganglionic eminences, and a disruption of axons traversing this region, indicate abnormalities in cell-type specification. The aberrant expression of Tuj-1, a marker of neuronal differentiation in the neuroepithelium, and Dlx, a marker of regional cell identity, in the ventricular zone in the MASH1 mutant brains suggest coordination of differentiation events is disrupted. In addition, the involvement of MASH1 in lateral inhibition processes that affect the development of these forebrain regions is implicated. Taken together, an essential role for MASH1 in the coordination of events required for correct cell-type specification and timing of differentiation during neural development in ventral forebrain regions is demonstrated.

The Phox2 homeodomain proteins are sufficient to promote the development of sympathetic neurons

The development of sympathetic neurons is controlled by a network of transcriptional regulators, including the paired homeodomain proteins Phox2a and Phox2b. To understand the role of Phox2 proteins in more detail, the effect of Phox2 overexpression was analysed in the avian peripheral nervous system. Phox2a expression in neural crest cultures elicited a strong increase in the number of sympathoadrenergic cells. Expression of Phox2a in the chick embryo promoted the generation of additional neurons expressing the noradrenergic marker genes DBH and TH, pan-neuronal genes SCG10 and NF160 and cholinergic genes ChAT and VAChT. Phox2a-induced neurons were found in ectopic locations such as dorsal root ganglia and peripheral nerve. Sympathoadrenergic development could be elicited in cultures of E5 dorsal root ganglia, demonstrating the presence of Phox2a-responsive cells in non-autonomic peripheral ganglia. Phox2b induced ectopic neurons in the chick embryo in the same way as Phox2a. These results show that Phox2 proteins are sufficient to promote sympathetic neuron generation and control, directly or indirectly, the expression of a large number of genes characteristic for sympathetic neurons.

Identification of dividing, determined sensory neuron precursors in the mammalian neural crest

Sensory and autonomic neurons of the vertebrate peripheral nervous system are derived from the neural crest. Here we use the expression of lineage-specific transcription factors as a means to identify neuronal subtypes that develop in rat neural crest cultures grown in a defined medium. Sensory neurons, identified by expression of the POU-domain transcription factor Brn-3.0, develop from dividing precursors that differentiate within 2 days following emigration from the neural tube. Most of these precursors generate sensory neurons even when challenged with BMP2, a factor that induces autonomic neurogenesis in many other cells in the explants. Moreover, BMP2 fails to prevent expression of the sensory-specific basic helix-loop-helix (bHLH) transcription factors neurogenin1, neurogenin2 and neuroD, although it induces expression of the autonomic-specific bHLH factor MASH1 and the paired homeodomain factor Phox2a in other cells. These data suggest that there are mitotically active precursors in the mammalian neural crest that can generate sensory neurons even in the presence of a strong autonomic-inducing cue. Further characterization of the neurons generated from such precursors indicates that, under these culture conditions, they exhibit a proprioceptive and/or mechanosensory, but not nociceptive, phenotype. Such precursors may therefore correspond to a lineally (Frank, E. and Sanes, J. (1991) Development 111, 895–908) and genetically (Ma, Q., Fode, C., Guillemot, F. and Anderson, D. J. (1999) Genes Dev. 13, in press) distinct subset of early-differentiating precursors of large-diameter sensory neurons identified in vivo.

Requirement for the homeobox gene Hb9 in the consolidation of motor neuron identity

The homeobox gene Hb9, like its close relative MNR2, is expressed selectively by motor neurons (MNs) in the developing vertebrate CNS. In embryonic chick spinal cord, the ectopic expression of MNR2 or Hb9 is sufficient to trigger MN differentiation and to repress the differentiation of an adjacent population of V2 interneurons. Here, we provide genetic evidence that Hb9 has an essential role in MN differentiation. In mice lacking Hb9 function, MNs are generated on schedule and in normal numbers but transiently acquire molecular features of V2 interneurons. The aberrant specification of MN identity is associated with defects in the migration of MNs, the emergence of the subtype identities of MNs, and the projection of motor axons. These findings show that HB9 has an essential function in consolidating the identity of postmitotic MNs.

Restriction of developmental potential during divergence of the enteric and sympathetic neuronal lineages

In the peripheral nervous system, enteric and sympathetic neurons develop from multipotent neural crest cells. While local environmental signals in the gut and in the region of the sympathetic ganglia play a role in the choice of cell fate, little is known about the mechanisms that underlie restriction to specific neuronal phenotypes. We investigated the divergence and restriction of the enteric and sympathetic neuronal lineages using immuno-isolated neural crest-derived cells from the gut and sympathetic ganglia. Analysis of neuronal and lineage-specific mRNAs and proteins indicated that neural crest-derived cells from the gut and sympathetic ganglia had initiated neuronal differentiation and phenotypic divergence by E14.5 in the rat. We investigated the developmental potential of these cells using expression of tyrosine hydroxylase as a marker for a sympathetic phenotype. Tyrosine hydroxylase expression was examined in neurons that developed from sympathetic and enteric neuroblasts under the following culture conditions: culture alone; coculture with gut monolayers to promote enteric differentiation; or coculture with dorsal aorta monolayers to promote noradrenergic differentiation. Both enteric and sympathetic neuroblasts displayed developmental plasticity at E14.5. Sympathetic neuroblasts downregulated tyrosine hydroxylase in response to signals from the gut environment and enteric neuroblasts increased expression of tyrosine hydroxylase when grown on dorsal aorta or in the absence of other cell types. Tracking of individual sympathetic cells displaying a neuronal morphology at the time of plating indicated that neuroblasts retained phenotypic plasticity even after initial neuronal differentiation had occurred. By E19.5 both enteric and sympathetic neuroblasts had undergone a significant loss of their developmental potential, with most neuroblasts retaining their lineage-specific phenotype in all environments tested. Together our data indicate that the developmental potential of enteric and sympathetic neuroblasts becomes restricted over time and that this restriction takes place not as a consequence of initial neuronal differentiation but during the period of neuronal maturation. Further, we have characterized a default pathway of adrenergic differentiation in the enteric nervous system and have defined a transient requirement for gut-derived factors in the maintenance of the enteric neuronal phenotype.

Hirschsprung disease and other enteric dysganglionoses

Hirschsprung disease has become a paradigm for multigene disorders because the same basic phenotype is associated with mutations in at least seven distinct genes. As such, the condition poses distinct challenges for clinicians, patients, diagnostic pathologists, and basic scientists, who must cope with the implications of this genetic complexity to comprehend the pathogenesis of the disorder and effectively manage patients. This review focuses on the anatomic pathology, genetics, and pathogenesis of Hirschsprung disease and related conditions. The nature and functions of "Hirschsprung disease genes" are examined in detail and emphasis is placed on the importance of animal models to this field. Where possible, potential uses and limitations of new data concerning molecular genetics and pathogenesis are discussed as they relate to contemporary medical practices.

The homeobox gene Phox2b is essential for the development of autonomic neural crest derivatives

The sympathetic, parasympathetic and enteric ganglia are the main components of the peripheral autonomic nervous system, and are all derived from the neural crest. The factors needed for these structures to develop include the transcription factor Mash1, the glial-derived neurotrophic factor GNDF and its receptor subunits, and the neuregulin signalling system, each of which is essential for the differentiation and survival of subsets of autonomic neurons. Here we show that all autonomic ganglia fail to form properly and degenerate in mice lacking the homeodomain transcription factor Phox2b, as do the three cranial sensory ganglia that are part of the autonomic reflex circuits. In the anlagen of the enteric nervous system and the sympathetic ganglia, Phox2b is needed for the expression of the GDNF-receptor subunit Ret and for maintaining Mash1 expression. Mutant ganglionic anlagen also fail to switch on the genes that encode two enzymes needed for the biosynthesis of the neurotransmitter noradrenaline, dopamine-beta-hydroxylase and tyrosine hydroxylase, demonstrating that Phox2b regulates the noradrenergic phenotype in vertebrates.

Innervation of the esophagus in mice that lack MASH1

The striated muscle of the esophagus differs from other striated muscle, because it develops by the transdifferentiation of smooth muscle, and the motor end plates receive a dual innervation from vagal (cholinergic) motor neurons and nitric oxide synthase (NOS)-containing enteric neurons. Mash1-/- mice have no enteric neurons in their esophagus and die within 48 hours of birth without milk in their stomachs (Guillemot et al. [1993] Cell 75:463-476). In this study, the innervation of the esophagus of newborn Mash1-/-, Mash1+/- and wild type mice was examined. There was no difference between Mash1-/-, Mash1+/-, and wild type mice in the transdifferentiation of the muscle and the development of nicotinic receptor clusters. However, there were significantly more cholinergic nerve terminals per motor end plate in Mash1-/- mice than Mash1+/- or wild type mice. Each of the Mash1-/- mice had fewer than 50 NOS neurons per esophagus, compared with approximately 3,000 in wild type mice. Newborn Mash1+/- mice also contained significantly fewer NOS neurons than wild type mice. In Mash1-/- mice, NOS nerve fibers were virtually absent from the external muscle but were present at the myenteric plexus. Unlike that of newborn wild type mice, the lower esophageal sphincter of Mash 1-/- mice lacked NOS nerve fibers; this may explain the absence of milk in the stomach. We conclude that 1) the transdifferentiation of the esophageal muscle and the development of the extrinsic innervation do not require enteric neurons or MASH1, 2) extrinsic NOS neurons only innervate the myenteric plexus.

Regulation of hippocampal neuronal differentiation by the basic helix-loop-helix transcription factors HES-1 and MASH-1

HES-1 is a vertebrate homologue of the Drosophila basic helix-loop-helix (bHLH) protein Hairy, a transcriptional repressor that negatively regulates neuronal differentiation. HES-1 expression in neuronal precursors precedes and represses the expression of the neuronal commitment gene MASH-1, a bHLH activator homologous to the proneural Achaete-Scute genes in Drosophila. Down-regulation of HES-1 expression in developing neuroblasts may be necessary for the induction of a regulatory cascade of bHLH activator proteins that controls the commitment and progression of neuronal differentiation. Here we show that the differentiation of embryonic day-17 rat hippocampal neurons in culture was coincident with a decline in HES-1 expression and DNA binding. Therefore, we examined the effect of forced expression of HES-1 and MASH-1 upon nerve growth factor (NGF) -induced differentiation in TrkA transfected hippocampal neurons. Expression of HES-1 inhibited both the intrinsic and NGF-induced neurite outgrowth, whereas MASH-1 expression increased neurite outgrowth. Strikingly, the increased hippocampal differentiation observed with MASH-1 expression is completely blocked by coexpression of HES-1. Furthermore, both wild-type HES-1 and a non-DNA binding mutant of HES-1 repressed MASH-1-dependent transcription activation. These results suggest that down-regulation of HES-1 is necessary for autonomous, growth factor-induced and MASH-1-activated hippocampal differentiation.

Mutations in the zebrafish unmask shared regulatory pathways controlling the development of catecholaminergic neurons

The mechanism by which pluripotent progenitors give rise to distinct classes of mature neurons in vertebrates is not well understood. To address this issue we undertook a genetic screen for mutations which affect the commitment and differentiation of catecholaminergic (CA) [dopaminergic (DA), noradrenergic (NA), and adrenergic] neurons in the zebrafish, Danio rerio. The identified mutations constitute five complementation groups. motionless and foggy affect the number and differentiation state of hypothalamic DA, telencephalic DA, retinal DA, locus coeruleus (LC) NA, and sympathetic NA neurons. The too few mutation leads to a specific reduction in the number of hypothalamic DA neurons. no soul lacks arch-associated NA cells and has defects in pharyngeal arches, and soulless lacks both arch-associated and LC cell groups. Our analyses suggest that the genes defined by these mutations regulate different steps in the differentiation of multipotent CA progenitors. They further reveal an underlying universal mechanism for the control of CA cell fates, which involve combinatorial usage of regulatory genes.

FMRFamide neuropeptides and neuropeptide-associated enzymes in Drosophila

To review the histochemistry of neuropeptide transmitters system in insects, this chapter focuses on the biology of FMRFamide-related neuropeptides in Drosophila. dFMRFamide expression is limited to a small number of neurons that present a complex spatial pattern and whose functions appear heterogeneous. The neuropeptide is first expressed by a few neurons in late stage embryos, then dynamically in as many as 44 neurons in the larval CNS. This review describes histochemical procedures to evaluate this neuronal phenotype and its regulation, including descriptions of promoter activity, and RNA and peptide distributions. To evaluate the use of peptidergic transmitters on a broad scale, I also review experiments in Drosophila studying enzymes necessary for neuropeptide biosynthesis, and in particular, histochemical studies of an enzyme responsible for peptide alpha-amidation.

Early specification of sensory neuron fate revealed by expression and function of neurogenins in the chick embryo

The generation of sensory and autonomic neurons from the neural crest requires the functions of two classes of basic helix-loop-helix (bHLH) transcription factors, the Neurogenins (NGNs) and MASH-1, respectively (Fode, C., Gradwohl, G., Morin, X., Dierich, A., LeMeur, M., Goridis, C. and Guillemot, F. (1998) Neuron 20, 483–494; Guillemot, F., Lo, L.-C., Johnson, J. E., Auerbach, A., Anderson, D. J. and Joyner, A. L. (1993) Cell 75, 463–476; Ma, Q., Chen, Z. F., Barrantes, I. B., de la Pompa, J. L. and Anderson, D. J. (1998 Neuron 20, 469–482). We have cloned two chick NGNs and found that they are expressed in a subset of neural crest cells early in their migration. Ectopic expression of the NGNs in vivo biases migrating neural crest cells to localize in the sensory ganglia, and induces the expression of sensory neuron-appropriate markers in non-sensory crest derivatives. Surprisingly, the NGNs can also induce the expression of multiple pan-neuronal and sensory-specific markers in the dermomyotome, a mesodermal derivative. Taken together, these data suggest that a subset of neural crest cells may already be specified for a sensory neuron fate early in migration, as a consequence of NGN expression.

Cellular and molecular determinants of sympathetic neuron development

The development of the sympathetic nervous system can be divided into three overlapping stages. First, the precursors of sympathetic neurons arise from undifferentiated neural crest cells that migrate ventrally, aggregate adjacent to the dorsal aorta, and ultimately differentiate into catecholaminergic neurons. Second, cell number is refined during a period of cell death when neurotrophic factors determine the number of neuronal precursors and neurons that survive. The final stage of sympathetic development is the establishment and maturation of synaptic connections, which for sympathetic neurons can include alterations in neurotransmitter phenotype. Considerable progress has been made recently in elucidating the cellular and molecular mechanisms that direct each of these developmental decisions. We review the current understanding of each of these, focusing primarily on events in the peripheral nervous system of rodents.

Deconstructing cell determination: proneural genes and neuronal identity

Vertebrates express scores of bHLH proteins during neural development. Earlier studies inspired by the established role of "proneural" genes in fly neurogenesis, as well as by the vertebrate bHLH myogenic program, focused on the reconstruction of bHLH gene cascades, which are thought to control successive steps leading to neuronal differentiation. Little attention has been paid thus far to the relationship between the diversity of neural bHLH genes and the diversity of neuronal phenotypes. This article reviews recent evidence that, akin to their fly counterparts, vertebrate neural bHLH genes probably confer not only "generic" neuronal properties, but also neuronal type-specific properties, inextricably linking neural determination and the specification of neuronal identity. We also speculate on the relations between positional information and gene activity, and on the evolutionary significance of the diversity of bHLH genes.

Hoxa2 and Hoxb2 control dorsoventral patterns of neuronal development in the rostral hindbrain

Little is known about how the generation of specific neuronal types at stereotypic positions within the hindbrain is linked to Hox gene-mediated patterning. Here, we show that during neurogenesis, Hox paralog group 2 genes control both anteroposterior (A-P) and dorsoventral (D-V) patterning. Hoxa2 and Hoxb2 differentially regulate, in a rhombomere-specific manner, the expression of several genes in broad D-V-restricted domains or narrower longitudinal columns of neuronal progenitors, immature neurons, and differentiating neuronal subtypes. Moreover, Hoxa2 and Hoxb2 can functionally synergize in controlling the development of ventral neuronal subtypes in rhombomere 3 (r3). Thus, in addition to their roles in A-P patterning, Hoxa2 and Hoxb2 have distinct and restricted functions along the D-V axis during neurogenesis, providing insights into how neuronal fates are assigned at stereotypic positions within the hindbrain.

Specification of neurotransmitter identity by Phox2 proteins in neural crest stem cells

We have investigated the specification of noradrenergic neurotransmitter identity in neural crest stem cells (NCSCs). Retroviral expression of both wild-type and dominant-negative forms of the paired homeodomain transcription factor Phox2a indicates a crucial and direct role for this protein (and/or the closely related Phox2b) in the regulation of endogenous tyrosine hydroxylase (TH) and dopamine-beta hydroxylase (DBH) gene expression in these cells. In collaboration with cAMP, Phox2a can induce expression of TH but not of DBH or of panneuronal genes. Phox2 proteins are, moreover, necessary for the induction of both TH and DBH by bone morphogenetic protein 2 (BMP2) (which induces Phox2a/b) and forskolin. They are also necessary for neuronal differentiation. These data suggest that Phox2a/b coordinates the specification of neurotransmitter identity and neuronal fate by cooperating environmental signals in sympathetic neuroblasts.

Transcription factors Mash-1 and Prox-1 delineate early steps in differentiation of neural stem cells in the developing central nervous system

Like other tissues and organs in vertebrates, multipotential stem cells serve as the origin of diverse cell types during genesis of the mammalian central nervous system (CNS). During early development, stem cells self-renew and increase their total cell numbers without overt differentiation. At later stages, the cells withdraw from this self-renewal mode, and are fated to differentiate into neurons and glia in a spatially and temporally regulated manner. However, the molecular mechanisms underlying this important step in cell differentiation remain poorly understood. In this study, we present evidence that the expression and function of the neural-specific transcription factors Mash-1 and Prox-1 are involved in this process. In vivo, Mash-1- and Prox-1-expressing cells were defined as a transient proliferating population that was molecularly distinct from self-renewing stem cells. By taking advantage of in vitro culture systems, we showed that induction of Mash-1 and Prox-1 coincided with an initial step of differentiation of stem cells. Furthermore, forced expression of Mash-1 led to the down-regulation of nestin, a marker for undifferentiated neuroepithelial cells, and up-regulation of Prox-1, suggesting that Mash-1 positively regulates cell differentiation. In support of these observations in vitro, we found specific defects in cellular differentiation and loss of expression of Prox-1 in the developing brain of Mash-1 mutant mice in vivo. Thus, we propose that induction of Mash-1 and Prox-1 is one of the critical molecular events that control early development of the CNS.

Transcriptional control of neurotransmitter phenotype

The specification of neurotransmitter phenotype is an important aspect of neuronal fate determination. Recent studies have begun to define essential transcriptional regulators involved in controlling the mode of neurotransmission in vertebrates and invertebrates, and to examine their regulation by cell-extrinsic factors. An emerging concept is that the control of transmitter choice is intimately linked to that of other aspects of the neuronal phenotype.

Cell type-specific regulation of the Drosophila FMRF-NH2 neuropeptide gene by Apterous, a LIM homeodomain transcription factor

We describe the direct and cell-specific regulation of the Drosophila FMRFa neuropeptide gene by Apterous, a LIM homeodomain transcription factor. dFMRFa and Apterous are expressed in partially overlapping subsets of neurons, including two of the seventeen dFMRFa cell types, the Tv neuroendocrine cells and the SP2 interneurons. Apterous contributes to the initiation of dFMRFa expression in Tv neurons, but not in those dFMRFa neurons that do not express Apterous. Apterous is not required for Tv neuron survival or morphological differentiation. Apterous contributes to the maintenance of dFMRFa expression by postembryonic Tv neurons, although the strength of its regulation is diminished. Apterous regulation of dFMRFa expression includes direct mechanisms, although ectopic Apterous does not induce ectopic dFMRFa. These findings show that, for a subset of neurons that share a common neurotransmitter phenotype, the Apterous LIM homeoprotein helps define neurotransmitter expression with very limited effects on other aspects of differentiation.

Growth factor synergism and antagonism in early neural crest development

This review article focuses on data that reveal the importance of synergistic and antagonistic effects in growth factor action during the early phases of neural crest development. Growth factors act in concert in different cell lineages and in several aspects of neural crest cell development, including survival, proliferation, and differentiation. Stem cell factor (SCF) is a survival factor for the neural crest stem cell. Its action is neutralized by neurotrophins, such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) through apoptotic cell death. In contrast, SCF alone does not support the survival of melanogenic cells (pigment cell precursors). They require the additional presence of a neurotrophin (NGF, BDNF, or NT-3). Fibroblast growth factor-2 (FGF-2) is an important promoter of proliferation in neuronal progenitor cells. In neural crest cells, fibroblast growth factor treatment alone does not lead to cell expansion but also requires the presence of a neurotrophin. The proliferative stimulus of the fibroblast growth factor - neurotrophin combination is antagonized by transforming growth factor beta-1 (TGFbeta-1). Moreover, TGFbeta-1 promotes the concomitant expression of neuronal markers from two cell lineages, sympathetic neurons and primary sensory neurons, indicating that it acts on a pluripotent neuronal progenitor cell. Moreover, the combination of FGF-2 and NT3, but not other neurotrophins, promotes expression or activation of one of the earliest markers expressed by presumptive sympathetic neuroblasts, the norepinephrine transporter. Taken together, these data emphasize the importance of the concerted action of growth factors in neural crest development at different levels and in several cell lineages. The underlying mechanisms involve growth-factor-induced dependence of the cells on other factors and susceptibility to growth-factor-mediated apoptosis.Key words: neural crest, melanocyte, stem cell factor, neurotrophin-3, transforming growth factor-beta1, apoptosis, norepinephrine transporter.

Specification of motor neuron identity by the MNR2 homeodomain protein

Sonic hedgehog signaling controls the differentiation of motor neurons in the ventral neural tube, but the intervening steps are poorly understood. A differential screen of a cDNA library derived from a single Shh-induced motor neuron has identified a novel homeobox gene, MNR2, expressed by motor neuron progenitors and transiently by postmitotic motor neurons. The ectopic expression of MNR2 in neural cells initiates a program of somatic motor neuron differentiation characterized by the expression of homeodomain proteins, by neurotransmitter phenotype, and by axonal trajectory. Our results suggest that the Shh-mediated induction of a single transcription factor, MNR2, is sufficient to direct somatic motor neuron differentiation.

A single rostrocaudal colonization of the rodent intestine by enteric neuron precursors is revealed by the expression of Phox2b, Ret, and p75 and by explants grown under the kidney capsule or in organ culture

The colonization of the rodent gastrointestinal tract by enteric neuron precursors is controversial due to the lack of specific cellular markers at early stages. The transcription factor, Phox2b, is expressed by enteric neuron precursors (Pattyn et al. Development 124, 4065-4075, 1997). In this study, we have used an antiserum to Phox2b to characterize in detail the spatiotemporal expression of Phox2b in the gastrointestinal tract of adult mice and embryonic mice and rats. In adult mice, all enteric neurons (labeled with neuron-specific enolase antibodies), and a subpopulation of glial cells (labeled with GFAP antibodies), showed immunoreactivity to Phox2b. In embryonic mice, the appearance of Phox2b-immunoreactive cells was mapped during development of the gastrointestinal tract. At Embryonic Days 9.5-10 (E9.5-10), Phox2b-labeled cells were present only in the stomach, and during subsequent development, labeled cells appeared as a single rostrocaudal wave along the gastrointestinal tract; at E14 Phox2b-labeled cells were present along the entire length of the gastrointestinal tract. Ret and p75 have also been reported to label migratory-stage enteric neuron precursors. A unidirectional, rostral-to-caudal colonization of the gastrointestinal tract of embryonic mice by Ret- and p75-immunoreactive cells was also observed, and the locations of Ret- and p75-positive cells within the gut were very similar to that of Phox2b-positive cells. To verify the location of enteric neuron precursors within the gut, explants from spatiotemporally defined regions of embryonic intestine, 0.3-3 mm long, were grown in the kidney subcapsular space, or in catenary organ culture, and examined for the presence of neurons. The location and sequence of appearance of enteric neuron precursors deduced from the explants grown under the kidney capsule or in organ culture was very similar to that seen with the Phox2b, Ret, and p75 antisera. Previous studies have mapped the rostrocaudal colonization of the rat intestine by enteric neuron precursors using HNK-1 as a marker. In the current study, all HNK-1-labeled cells in the gastrointestinal tract of rat embryos showed immunoreactivity to Phox2b, but HNK-1 cells comprised only a small subpopulation of the Phox2b-labeled cells. In addition, in rats, Phox2b-labeled cells were present in advance of (more caudal to) the most caudal HNK-1-labeled cells by 600-700 microm in the hindgut at E15. We conclude that the neural crest cell population that arises from the vagal level of the neural axis and that populates the stomach, midgut, and hindgut expresses Phox2b, Ret, and p75. In contrast, the sacral-level neural crest cells that populate the hindgut either do not express, or show a delayed expression of, all of the known markers of vagal- and trunk-level neural crest cells.

AP1 proteins mediate the cAMP response of the dopamine beta-hydroxylase gene

Neurotransmitter biosynthesis is regulated by environmental stimuli, which transmit intracellular signals via second messengers and protein kinase pathways. For the catecholamine biosynthetic enzymes, dopamine beta-hydroxylase and tyrosine hydroxylase, regulation of gene expression by cyclic AMP, diacyl glycerol, and Ca2+ leads to increased neurotransmitter biosynthesis. In this report, we demonstrate that the cAMP-mediated regulation of transcription from the dopamine beta-hydroxylase promoter is mediated by the AP1 proteins c-Fos, c-Jun, and JunD. Following treatment of cultured cells with cAMP, protein complexes bound to the dopamine beta-hydroxylase AP1/cAMP response element element change from consisting of c-Jun and JunD to include c-Fos, c-Jun, and JunD. The homeodomain protein Arix is also a component of this DNA-protein complex, binding to the adjacent homeodomain recognition sites. Transfection of a dominant negative JunD expression plasmid inhibits cAMP-mediated expression of the dopamine beta-hydroxylase promoter construct in PC12 and CATH.a cells. In addition to the role of c-Fos in regulating dopamine beta-hydroxylase gene expression in response to cAMP, a second pathway, involving Rap1/B-Raf is involved. These experiments illustrate an unusual divergence of cAMP-dependent protein kinase signaling through multiple pathways that then reconverge on a single element in the dopamine beta-hydroxylase promoter to elicit activation of gene expression.

Control of noradrenergic differentiation and Phox2a expression by MASH1 in the central and peripheral nervous system

Mash1, a mammalian homologue of the Drosophila proneural genes of the achaete-scute complex, is transiently expressed throughout the developing peripheral autonomic nervous system and in subsets of cells in the neural tube. In the mouse, targeted mutation of Mash1 has revealed a role in the development of parts of the autonomic nervous system and of olfactory neurons, but no discernible phenotype in the brain has been reported. Here, we show that the adrenergic and noradrenergic centres of the brain are missing in Mash1 mutant embryos, whereas most other brainstem nuclei are preserved. Indeed, the present data together with the previous results show that, except in cranial sensory ganglia, Mash1 function is essential for the development of all central and peripheral neurons that express noradrenergic traits transiently or permanently. In particular, we show that, in the absence of MASH1, these neurons fail to initiate expression of the noradrenaline biosynthetic enzyme dopamine beta-hydroxylase. We had previously shown that all these neurons normally express the homeodomain transcription factor Phox2a, a positive regulator of the dopamine beta-hydroxylase gene and that a subset of them depend on it for their survival. We now report that expression of Phox2a is abolished or massively altered in the Mash1−/− mutants, both in the noradrenergic centres of the brain and in peripheral autonomic ganglia. These results suggest that MASH1 controls noradrenergic differentiation at least in part by controlling expression of Phox2a and point to fundamental homologies in the genetic circuits that determine the noradrenergic phenotype in the central and peripheral nervous system.